A period correct 1999 PC Pentium III Coppermine either Slot 1 (SECC2) or Socket 370 (FC-PGA); 100 MHz FSB modles include: 500, 550, 600, 650, 700, 750, 800, 850, 900, 1000Mhz; I don't think they made the 1.1Ghz in Slot-1 though I could be wrong. As for 133 MHz FSB your options are: 533, 600, 667, 733, 800, 866, 933, 1000, 1133 MHz, RAM would be 128-256mb PC-100/PC-133 RAM, and a Voodoo 3 or GeForce 256 [and since he's wanting both Glide AND D3D, he's stuck with the Voodoo 3 which IMHO was no slouch, but T&L really kills it], the most commonly used soundcards in that era were the Sound Blaster 16 (yes many people still used the ISA version), Sound Blaster Live!, or Aureal Vortex/Vortex 2... i'd personally go with a Vortex 2 because those cards were AWESOME!
A period correct 2000 PC would be simular to above except the video card options are more... GeForce 2 Ultra was "king", Radeon R100 (7xxx) series, Voodoo 5 [this will once again get him Glide and D3D, and is faster than Voodoo 3, but the GeForce 2 Ultra walks all over it in raw horsepower], or Power VR Series 3 [aka Kyro... which is mediocre performance]. Every other spec was basically the same.
If he wants both worlds... get a GeForce 2 Ultra (AGP; D3D support) and SLI-Voodoo 2's (PCI; full Glide support) [as far as I know that combo will work, because most games ask which display adapter to use].
DOS Gaming System: MS-DOS, AMD K6-III+ 400/ATZ@600Mhz, ASUS P5A v1.04 Motherboard, 32 MB RAM, 17" CRT monitor, Diamond Stealth 64 3000 4mb PCI, SB16 [CT1770], Roland MT-32 & Roland SC-55, 40GB Hard Drive, 3.5" Floppy Drive.